The present invention relates to the field of providing suitable atmospheres for heat treating furnaces, or other high-temperature environments, having operating temperatures of the order of about 1500.degree. F., or more.
One example of a heat treating process is shown in U.S. Pat. No. 4,139,375, dealing with sintering of compact powder metal. The atmosphere used in this process prevents oxidation of the parts, among other functions. The furnace atmosphere can also be used to supply carbon to a metal, when the process requires that the level of carbon in the treated metal be precisely maintained.
In the heat treating of metals, it is known to use carrier gases containing hydrogen, carbon monoxide, and nitrogen, and possibly small amounts of other substances such as methane. One method of producing such a gas, in the prior art, has been to inject liquid methanol (CH.sub.3 OH) into the furnace chamber. The methanol dissociates in the furnace to yield hydrogen and carbon monoxide. However, it is undesirable to inject liquid methanol directly into a furnace. Methanol is very corrosive, and can eat away the metal parts of the furnace, and also the metal belts used to transport workpieces through the furnace. It also corrodes the metal parts of flow measuring equipment located in and around the furnace. Methanol is particularly destructive of aluminum, but it also corrodes other metals, greatly increasing the cost of maintaining the system.
In some systems of the prior art, methanol is vaporized, but not dissociated, outside the furnace. These systems suffer from the problem of condensation of methanol along the transport line. The corrosion problems discussed in reference to the furnace components apply equally to the piping outside the furnace. Also, the combustible content of the furnace atmosphere cannot be precisely controlled in such systems.
Another type of method of generating an atmosphere for a heat treating furnace is shown in U.S. Pat. No. 4,249,965. In the latter patent, nitrogen is mixed with methanol, and the methanol is vaporized, outside the furnace. The vaporized mixture is heated in the presence of a catalyst, also outside the furnace, and the methanol reacts to form hydrogen and carbon monoxide. However, the percentages of nitrogen, hydrogen, and carbon monoxide are fixed by the particular catalytic reaction, and there is no way of varying and controlling the ratio of combustibles to non-combustibles in the furnace atmosphere. Furthermore, while catalysts for dissociation of methanol have the advantage of enabling the dissociation to occur at a low temperature, such catalysts are relatively expensive.
There are many other examples of systems and methods of producing atmospheres for use in heat treating applications. U.S. Pat. No. 4,445,945 discloses a carrier gas of carbon monoxide, hydrogen, and nitrogen, produced from an initial mixture of methanol and nitrogen. U.S. Pat. No. 4,285,742 shows a carrier gas formed from a mixture of nitrogen, water, and methanol injected directly into the heat treating furnace. U.S. Pat. No. 4,450,017 discloses another process wherein the furnace atmosphere includes nitrogen, methanol, and carbon dioxide.
Additional references dealing with atmospheres for heat treating furnaces, for various applications, are U.S. Pat. Nos. 4,049,472, 3,986,900, 4,386,972, 4,372,790, 4,153,485, 4,106,931, 4,069,071, 4,317,687, 4,236,941, 4,175,986, 4,108,693, and 3,891,473.
The present invention discloses a system and method for generating a controlled atmosphere for a high-temperature process. In particular, the invention provides a system for mixing dissociated methanol with nitrogen, outside of the furnace, regulating the relative amounts of combustibles in the mixture, and injecting the mixture into the furnace. The methanol is vaporized and dissociated outside of the furnace, so no liquid ever enters the furnace. The system thereby avoids the corrosion problems due to liquid methanol. The system employs a heater, of conventional design, as a dissociation reactor, and does not depend on the existence of catalysts for the dissociation reaction. The process of the present invention also saves energy, in comparison with methods of the prior art, and provides greater control over the components in the heat treating atmosphere.
The present invention also includes a system wherein regulated atmospheres may be provided to a plurality of furnaces, each atmosphere having a separately regulated proportion of combustible components.